Samantha 'Sam' Star is shown to be highly aggressive by her clarity. This can be true as she is easily provoked by any person she doesn't recognize. However, she expresses a brotherly bond with her younger brother and defends him from the other neighbors, SpongeBob and Squidward, when she mistaken physical contact as aggression. In addition to her supportive behavior, her level of intelligence is relatively lower than Patrick's as she speaks incomplete language and has no regards for other persons.
It's possible that since her first appearance, she became Squidward's sworn enemy due to the fact that they nearly killed each other in her debut.